1. Field of the Invention
The invention relates to a coordinates input apparatus and, more particularly, to a coordinates input apparatus for correcting accuracy by using, for instance, a propagation delay time of a surface acoustic wave.
2. Related Background Art
Hitherto, the following method has been used as a method of correcting an origin of a coordinates input apparatus using a surface acoustic wave. For instance, when sensors S.sub.0 to S.sub.3 are arranged on a vibration transmission plate as shown in FIG. 6, origin correction coefficients (n.sub.n), distances (d.sub.n) from a point P (x, y) to the sensors, an x coordinate value (x) of the point P, and a y coordinate value (y) of the point P are respectively obtained by the following equations (1) to (4) on the assumption that known group delay times and phase delay times from an origin (point O in FIG. 6) to the sensors are set to tgz.sub.n and tpz.sub.n (n=0 to 3) and group delay times and phase delay times from an arbitrary point P (x, y) on the vibration transmission plate to the sensors are respectively set to tg.sub.n and tp.sub.n (n=0 to 3). EQU n.sub.n =[(v.sub.g (tg.sub.n -tgz.sub.n)-v.sub.p (tp.sub.n -tpz.sub.n))/.lambda.+1/N] (1) EQU d.sub.n =n.sub.n .multidot..lambda.+v.sub.p (tp.sub.n -tpz.sub.n)(2) EQU x=(d.sub.0.sup.2 -d.sub.1.sup.2)/2X+d.sub.0 /Dz.sub.0 -d.sub.1 /Dz.sub.1( 3) EQU y=(d.sub.2.sup.2 -d.sub.3.sup.2)/2Y+d.sub.2 /Dz.sub.2 -d.sub.3 /Dz.sub.3( 4)
where, vg: group velocity
vp: phase velocity PA1 .lambda.: wavelength of vibration wave PA1 X: distance between the sensors S.sub.0 and S.sub.1 PA1 Y: distance between the sensors S.sub.2 and S.sub.3 PA1 Dz.sub.n : distance between the sensor and the zero point PA1 N.noteq.0: correction term to form an integer PA1 []: to form an integer by omitting the figures below a decimal point
That is, an origin correction is performed by considering group delay time and phase delay time which are measured by each sensor.
In the conventional method, however, as will be also obviously understood from the equation (1), when the position of the point P whose coordinate values are detected is far from the point O, a difference between the delay time from the point O to the sensor and the delay time from the point P to the sensor increases, so that the value of n.sub.n -[n.sub.n ] also increases in association with such an increase in delay time difference. In addition, since the value of n.sub.n is rounded to an integer, for instance, when N in the equation (1) assumes 2 and the value of n.sub.n is rounded to the nearest whole number, the value of n.sub.n -[n.sub.n ] discontinuously changes by only .+-.1 at the points where the position of the point P whose coordinate values are detected is fairly far from the point O as shown in FIG. 7, that is, at points a and b. For this above reason, when a constant of the velocity or the like changes due to a change in component elements of the apparatus such as a variation in thickness of the vibration transmission plate or the like or a change in external environment such as a temperature change or the like, there is a case where the value of [n] is erroneously calculated by .+-.1 and the accuracy is deteriorated at positions near the points a and b.
Further, in the conventional apparatus, only one origin correction coefficient (n.sub.n) is provided for each of the sensors, so that the above deterioration of the accuracy cannot be compensated.
For instance, as a deviation amount of the group velocity from the phase velocity is large, the points a and b approach the point O and points such as to cause the deterioration of the accuracy appear within a range of the vibration transmission plate. Even when such a deviation amount is small, in the case where the size of the apparatus enlarges (in the case where the size of the vibration transmission plate is large), even if the points a and b are sufficiently away from the point O, they can exist in the vibration transmission plate. Consequently, there is a possibility such that such an accuracy deterioration occurs.
As another factor to deteriorate the accuracy, further, there can be mentioned an offset deviation of a whole region as shown in, e.g., FIG. 8 which occurs by a change in delay amount on the circuit or propagating path or by a change in delay amount due to the inclination of the pen or the like. In FIG. 8, the offset deviation occurs on the + side, an allowance of the accuracy correction on the + side decreases, and a possibility such that the accuracy deterioration occurs for a fluctuation from the outside is high. There is also a case where such an offset fluctuates during the operation of the apparatus. The offset cannot be eliminated by the ordinary O point correction.